Photoradiation therapy (in situ activation by visible light of hematoporphyrin derivative) is effective against many solid tumors in animal and man. We are studying both practical means of applying this therapy more effectively and basic mechanisms involved in its accumulation in tumors and photodynamic action. In particular, we are defining various means of light delivery, especially using a laser-fiber optic system, attempting to determine an optimum treatment schedule (drug dose, time interval, light intensity, etc.), examining tissue and cellular repair, defining cellular targets, defining the oxygen requirement and looking at various combinations with other modalities such as radiation and chemotherapy.